1. Field of the Invention
The invention relates to a method and an arrangement for magnetic digital recording with high frequency biasing.
2. Description of the Prior Art
The magnetic recording of a data signal consists basically in creating a write current in a magnetic head which produces on one face of a magnetic support such as a tape, disc or drum a residual magnetization representative of the initial data.
In the case of an electric analog data signal, for example, a signal whose amplitude is representative of sound from an acoustic transducer, it is evident that the residual magnetization created by the write current should faithfully reproduce, in time, the amplitude of the data signal. This is universally obtained by superimposing on the data signal a high frequency, constant amplitude, alternating current signal which constitutes a magnetic biasing. Put briefly, the curve of first magnetization of the magnetic material of the recording carrier starts with a quadratically curving part, extends in a linear part and ends in a part which curves in to end in saturation. Without the magnetic biasing current, the recording of the data signal would correspond to making a more or less large excursion in the curve of the first magnetization from its origin and at each point of the recording carrier according to the level of amplitude of the write signal. The quadratic part of the curve would always be involved so that the recorded signal would present a high rate of distortion. The superposition on the data signal of a high frequency, constant amplitude, alternating current signal constitutes a magnetic biasing in this sense that the excursions can then be made in the linear part of the curve of first magnetization. Furthermore, the high frequency magnetic biasing leaves the recording carrier non-magnetized in the absence of the data signal, the more so since the corresponding wavelength of the biasing current is inferior to the power of resolution of the read head, which power is essentially dependent on the value of the read air gap. On the other hand, although a direct current magnetic biasing is also possible, it would place the recorded carrier, in the absence of the data signal, in a permanently magnetized state which would be reflected in the read signal by a great background noise.
A binary coded digital data signal indicates successively, at a given recurrence frequency, the value 0 or 1 of a data bit. This signal comprises, therefore, two correlative components--a repeated series of instants and a series of corresponding binary values--which the residual magnetization should normally translate faithfully. To do this, it is attempted to represent at least one of the two binary values by a magnetic flux transmission determined as a function of a selected code, occurring at a precise corresponding instant.
In current practice, magnetic transitions are advantageously inversions of biasing of the residual field, designed to make this field change between two predetermined positive and negative biasing levels of the magnetic material of the recording carrier. The result of this is to create in this carrier a set of magnets placed end-to-end, with adjacent poles of the same kind, and of length corresponding to the time interval separating two transitions confirming to the type of coding chosen. By convention, a reversal of the residual field form a negative level to a positive level of biasing will be called a positive transition, the reversal in the opposite direction being therefore a negative transition.
Among the types of coding most used is that called NRZ1 (Non Return to Zero for bits of value "1"). In the NRZ1 code only the 1 values are represented by magnetic transitions, independently of the direction of these transitions. In another popular code called "coded phase", the two binary values correspond respectively to the positive and negative transitions. As will be seen later, the invention is unaffected by the method of coding chosen.
Various problems arise relative to the fidelity of recording and reading of the other component of the digital data signal relative to the instants at which the transitions should have taken place.
It has been noted previously that the binary data is translated on the recording carrier as a series of magnets placed end-to-end, of which the adjacent poles are of the same name and translate the existence of a transition. The read current produced by the read head during passage of two adjacent semi-magnets is, therefore, in the form of a clock or a bell-shaped curve, of which the peak corresponds to the transition, since the variation of magnetic flux in the read winding is greatest during passage of the two neighboring poles of the two magnets before the air gap of the read head. However, when two transitions are very close together (which is the case with high recording densitities), the successive curves run into each other or combine so that the current peaks are offset from the actual transitions. This phenomenon, more generally known as peak shift, increases with the frequency of transitions so that, for high recording densitities, the peaks can be shifted by up to about one third of the smallest space which can separate two transitions. The decoding circuits must therefore be very active, the more so since to this shifting variations in the speed of travel of the recording carrier are added. Various attempts have therefore been made with a view to reducing the size of the peak shift.
Results have been obtained in this direction by using a digital recording signal similar to an analogue recording signal. Experience has in fact shown a reduction in peak shift for high write densities, about around 200 inversions of flux per millimeter, with a composite recording signal formed by the superposition of a high frequency, constant amplitude, magnetic biasing alternating signal on the digital coded data signal.
In this composite recording signal, each transition is represented by a difference in peak amplitudes of the same sign as two neighboring half-waves of the biasing signal which are present respectively before and after the instant of transition corresponding to the data signal. Thus, the high frequency biasing is of interest from the moment when these two alternations are separated by a fixed time interval, theoretically corresponding to the period of the biasing signal and resulting in a suppression of the peak shift. However, in alternating biasing, digital recording devices of the prior art, this time interval can deviate unequally and erratically from the value of this period and can cause uncertainties and errors in decoding the signal registered by these devices. These deviations result from the random distribution of transitions in the coded digital data signal, the latter being therefore able to arise at any instant in a period of the magnetic biasing signal and act so that the superposition of the two signals is more or less favorable. The more favorable situation (zero deviation) occurs when there is a coincidence between a transition of a given sign and the peak amplitude of the same sign of the biasing signal. On the other hand, the deviation is maximum when the transition of a given sign occurs at the moment when the biasing signal reaches a peak amplitude of the opposite sign, in which case the following peak amplitude is delayed by about a half-wavelength of the biasing signal.
It follows that the size of the peak shift depends on the phase of the biasing signal with respect to the coded signal and that, if the shift is on average effectively reduced by the biasing, relatively high values can be obtained for certain transitions and very active circuits will be needed for the reading and decoding of signals recorded in this manner.
To avoid this peak shift, it would appear of interest to render the biasing signal synchronous, as regards frequency, with the clock for controlling the coded digital signal to be recorded. However, because the positive and negative transitions in the coded signal are distributed in a random manner, the phase that exists between each transition and the magnetic biasing signal remains uncertain, so that more or less favorable cases will still occur, as in the preceding case.
The increase in the frequency of the biasing signal with respect to the higher recurrence frequency of transitions diminishes the peak shift effect, but it is limited by the fact that it raises the electromagnetic losses in the materials forming the recording heads. Furthermore, the peak shift can be reduced by a judicious relationship between the frequency of the magnetic biasing signal and the clock frequency according to the rhythm at which the coding is effected. Nevertheless, an improved decoding device is necessary.
One solution to avoid the appearance of any unfavorable case during elaboration of a digital recording signal with alternating biasing is described in my co-pending application Ser. No. 156,368, corresponding to French application No. 79.15884. This solution consists in phase modulating relative to the transitions of the coded data signal, a standard, high frequency, original biasing signal, consisting of a sinusoidial signal or, more usually, a rectangular or square wave signal. In this connection, it is pointed out that a square wave signal is a pulse signal of which the cyclic relationship between the pulse duration and its period of recurrence is 0.5. A pulse signal with a cyclic ratio different from 0.5 is called a rectangular signal. More precisely this phase modulation is translated by a successive shift by 180.degree. of the original biasing signal from each appearance of the transitions of the coded data signal. Each transition thus reflects back, in the modulated biasing signal, by a doubling of the pulse or the correponding alternation of the original biasing signal, so that the superposition of the modulated biasing signal to the data signal is always produced in favorable conditions, i.e. not raising any problem for combination of signals bordering on variable and random delays in appearance of the recording signal transitions with respect to the transitions of the data signal.
The present invention profits from this method of recording to simplify the manufacture of read heads called "integrated" read heads, i.e. miniature heads obtained by depositing thin layers on common or individual substrates.
Magnetic writing of digital data on a recording carrier requires passage in a write coil of recording currents of sufficiently high intensities to advantageously produce fields of saturation of the magnetic material of the recording carrier capable of thus optimizing the residual fields on the recording carrier and facilitating reading of data as a result. Further, with the codes usually used for formation of the coded data signal, the random distribution of transitions means that the average value of the recording current is itself occasionally nil, which separates use of a current transformer downstream of the writing winding with a view to increasing the recording field with a winding which has a reduced number of turns (preferably one turn). The result was that the previous recording devices with conventional codes had to include recording heads with a winding composed of a relatively high number of turns (ordinarily of the order of 2.times.10 turns). The manufacture of a winding with several superposed turns in an integrated head presents many difficulties, which have given rise to numerous inventions, among which will be noted those described in French Pat. Nos. 2063693 and 2063694. A one turn winding is therefore desirable.
In this way, one solution requires a specific code enabling to obtain on average a nil value of the recording current. However, this method necessitates a reduction in the amount of useful data with respect to the amount of original data and further prohibits recording of normalized codes.